1. Field of the Invention
This invention relates to apparatus for producing a "hard" copy of a static video image displayed on a cathode ray tube (CRT), and more particularly to such apparatus which employ photography for image copying purposes.
2. Description Relative to the Prior Art
A well known approach to obtaining a "hard" color copy of a video image is to photograph the image while it is displayed on a color cathode ray tube (CRT). This approach requires a static image that can be achieved, for example, by electronically selecting a video signal representative of a single frame of a television picture and repeatedly applying the signal to the CRT. The static image could alternatively be produced from a videotape recorder or video disk system operating in a still-frame mode.
The color copy from this approach, however, is of poor image quality when compared with a direct photograph of the scene being televised or videotaped. It is known that the image quality can be improved by separating the color components of the video signal associated with a scene, viz. the red, green and blue video signals, and sequentially displaying the component video signals on a monochromatic CRT. The screen of the CRT is photographed through color filters, the colors of the filters being synchronized to the corresponding color video signal components being displayed on the CRT, thereby to expose the photographic film to the red, green and blue color components of the video image.
While the color separation method does provide improved image quality in comparison with photographing a color CRT, the image quality suffers from the appearance of "raster" lines in the photograph due to the scanning pattern of the CRT electron beam. The photographed image appears, therefore, to be composed of a number of parallel image lines instead of a continuous image.
One method known in the prior art to obscure the raster lines is to display the video image a multiple of times, offsetting the image vertically each time the image is displayed (see for example U.S. Pat. No. 4,360,805). This method requires a large number of exposures of the static image (typically sixteen) in order to obscure the raster lines sufficiently. In the color separation system previously described, this method would require an excessive number of exposures (sixteen for each color component) in order to produce a photograph of the video image.
Other art which may be of interest in relation to the present invention may be found in U.S. Pat. No. 4,437,121 and U.S. Pat. No. 4,225,929.